Compositions for coating of paper and paperboard are well known in the art and usually comprise a pigment component, binder components and other miscellaneous coating components such as f lubricants, stabilizers, dispersants, defoamers, biocides, preservatives and mixtures thereof.
Paperboard coating co-binders compositions may additionally comprise protein, such as soy protein or casein, as a co-binder, to improve glueability and coating structure sufficiently for the coated board to be useful in the manufacture of boxes, cartons and other items. Protein also enhances the hiding power of the coating, improving the appearance of the coated board. However, protein suffers from numerous disadvantages. Protein is difficult to handle requiring a cooking or make-down step at high temperature, normally with concentrated ammonia right before addition to the paperboard coating. Protein solutions are especially susceptible to spoilage from biological attack. Since the color and properties of the paper coating composition will deteriorate as a result of the bacterial attack on the protein binder, it is often not possible to prepare large quantities of these materials for long paper coating runs. Accordingly, there is a desire in the art to replace protein with a co-binder component that does not have the protein cooking and make-down requirements and related spoilage and odor issues.
It would thus be advantageous to replace protein co-binder with a synthetic polymer which has the hiding power, glueability, coating bulking ability, blocking resistance and sheet gloss properties without the associated spoilage, odor and make-down problems associated with protein. The present inventors have surprisingly discovered that synthetic, substantially water-soluble amphoteric polymers function as well or better than proteins as co-binders in paper and paperboard coating applications. Since the amphoteric (co)polymers of the invention are available in solution (substantially water-soluble), their use as co-binder does not require complicated make-down procedures typical of protein co-binders.
Furthermore, the co-binder of the invention is also suitable for use in cast-coated paper. Cast coating involves pressing a coated substrate against a highly polished heated surface and drying the coated substrate against the polished surface. The term “casting,” as used herein, is intended to mean the step in which a coated substrate is pressed and dried against the casting surface, regardless of the coated substrate's physical state. The three methods are wet casting, gel casting, and rewet casting. In each case, the surface of the cast-coated substrate exhibits the same gloss and smoothness as the highly polished casting surface.
Usually some combination of casein (a milk protein) with synthetic latex is used to achieve the correct coating composition suitable for cast-coated papers having an extremely high gloss, smoothness, and flexibility, pick resistance and drum release. However, the disadvantages of using casein are similar to those mentioned above for other proteins.
Thus the co-binder of the invention is also suitable as a casein replacement in cast coating.
Amphoteric (co)polymers are well known in the art. For example, U.S. Pat. No. 4,533,708 describes polyampholyte polymers. There is no suggestion to use these polymers as co-binders in paper and paperboard coatings.
Synthetic co-binders are known in the art. Specifically Hanciogullari, H., “Synthetic Cobinders and Thickeners,” in PIGMENT COATING AND SURFACE SIZING OF PAPER, ed. E. Lehtinen, Fapet OY © 2000 (chapter 15, pp. 219-239) discusses the use of copolymers of nonionic monomers and acrylic acid or methacrylic acid as useful for adjusting rheological properties and water retention in paperboard coatings. Lee, Y et al., Kami Pa Gikyoshi, Vol. 56 (4), 2002, pp 543-548 discuss the effectiveness of amphoteric latexes in binder migration and uneven binder distribution in coating layers on acidic base paper. These amphoteric latexes are insoluble in water.
Lee, Y et al., J. Ind. Eng. Chem, Vol 8, (5), 2002, pp. 443-453 discuss the electrokinetic behavior of anionic and amphoteric latexes with pigment particles and base paper.
U.S. Pat. No. 5,536,764 discloses binder materials in which vinyl acetate grafted starch dispersions and blends of cationic starch and polyvinyl acetate are used to replace protein as co-binder in paperboard coating formulations.
All of the examples above deal with latex or latex blends (insoluble in water), which behave very differently than amphoteric substantially water-soluble (co)polymers of the invention particularly during coating consolidation and drying.
U.S. Pat. No. 6,255,427 discloses amphoteric polymeric dispersions obtained in the presence of at least one starch or starch derivative. The formed starch and amphoteric polymer may be used as a binder in pigment-containing coatings. U.S. Pat. No. 3,598,623 and U.S. Pat. No. 3,884,853 disclose amphoteric starches for use as binders in paper or paperboard. Both patents disclose derivatized starches containing carboxyl and tertiary amine groups. These starch-based amphoterics also suffer from similar spoilage and discoloration issues as protein hydrosylates.
U.S. Published Application 2003/0016280 and 2003/0035932 disclose ink-receptive compositions. The compositions contain an amphoteric polymer, a water-soluble nonionic polymer and a polyalkylene glycol or silicone surfactant.
PCT Published Application 2003/037641 discloses an inkjet media comprising a polymeric composition derived from at least one betaine monomer.